The uncooled infrared sensor (hereafter referred to as infrared imaging element as well) is a device which absorbs condensed infrared rays by using an infrared detection unit having detection cells and converts radiant energy to an electric signal. The detection cell has an infrared absorption material and a thermoelectric conversion element, and thermally isolated from an external system. For implementing such a structure, a technique for forming a surface fine structure or a bulk fine structure is needed. Unlike the cooled infrared sensor, the uncooled infrared sensor has an advantage that it is inexpensive and the size can be made small.
For implementing thermal isolation from the external system, it is important to install detection cells in the vacuum and lower the thermal conductance of a support structure which connects the detection cells to a substrate physically and electrically. It is desirable to keep the atmosphere incorporating the infrared imaging element in the vacuum state in order to enhance the heat insulation of the infrared imaging element. As one technique for that purpose, it is disclosed in JP-A-10-132654 (KOKAI) to incorporate an infrared sensor in a dedicated case and perform vacuum sealing.
As another technique, it is disclosed in JP-A-2005-236159 (KOKAI) to mount an infrared sensor on a hermetically sealed package which has two plate-like members and in which a space surrounded by a joining portion and the two plate-like members is hermetically sealed. As compared with an existing technique of mounting an infrared sensor on a semiconductor package and joining and sealing a window material in a vacuum atmosphere, it is anticipated to mount an infrared sensor on a wafer level package in the respect that the throughput at the time of assembling is improved and the productivity is improved.
In the infrared sensor described in JP-A-2005-236159 (KOKAI) or other infrared sensors mounted on the wafer level package, the sensor substrate and a seal plate are joined typically in the longitudinal direction. In the infrared sensor, the seal plate needs to be formed of a material which transmits infrared rays, such as silicon, unlike a typical visible image sensor. Furthermore, as for joining the sensor substrate and the seal plate together, solder is provided in the joining portion and the solder is melted by heat in a vacuum atmosphere to perform joining.
For playing the role of the window material which transmits infrared rays, the material of the seal plate is determined uniquely to be, for example, silicon. This results in a problem that drastic cost down in the seal process is difficult. Furthermore, since the sensor substrate and the seal plate are joined together by thermal melting of solder causing a temperature rise of several hundred degrees, the infrared sensor is subject to great thermal stress. It becomes an important point in considering the reliability of the infrared sensor.